With recent rapid development of industry, organic synthetic high polymers have become applied to many uses, such as decorations, building materials, daily necessaries, constructions, buildings and heat-insulating materials in industry. Polystyrene is excellent in transparency, electric properties and thermoplasticity, and hence has often and much been used in the above-mentioned various fields.
Styrenic based polymers are prone to depolymerization when heated to elevated temperatures. The styrene monomer vapor, like most monomer vapors, is highly flammable. Consequently, great care must be exercised in the utilization of styrenic based polymers in applications such as electrical appliances, automotive components, electronic instruments, etc. since these components are frequently exposed to high temperatures and ignition sources.
Television cabinets are a case in point. As a consequence of the fire hazard, regulatory agencies have established flammability standards on the components of television sets which are scheduled to increase in rigidity over a several year period of time. Ordinary styrenic polymers possess the requisite physical and electrical properties but cannot meet flammability requirements.
There is a need for additives which can be combined with styrenic polymers to reduce their inherent flammability. These additives must not, however, compromise the other desirable properties of the polymers.
Typical fire retardant additives which have been used for polymers include halogenated aliphatic hydrocarbons, such as 1,1,2,2-tetrabromoethane, 1,2,3,4-tetrachloroethane, 1,2-dibromoethane (Japanese patent publication No. 5,739/62), 2-chloro-1,2,-3,4-tetrabromobutane (Japanese patent publication No. 20,216/66), chlorinated parrafin wax and the like, haloalkyl phosphates, such as tris-(2,3-dibromopropyl) phosphate (Japanese patent publication No. 6,788/58) and the like and acetals or ethers of 2,3-dibromopropanol-1 (Japanese patent publication No. 7,089/60) are known as flame-retarding agents for polystyrene.
Chlorinated and brominated aromatic compounds have tended to be more efficient flame retardants for styrenic polymers than have the halogenated aliphatics. Thus, because of increased compatibility and decreased volatility, hexabromobenzene, the brominated and chlorinated diphenyls and completely halogenated decabromodiphenyl ether or decachlorodiphenyl ether have been used extensively. This is illustrated by U.S. Pat. Nos. 2,022,634; 2,188,903; 3,072,728; 3,347,822 and 3,728,304. As with most halogenated compounds, and particularly in the use of aromatics, there has been considerable concern over the persistence and possible toxicological effects. Also, these additives tend to cause yellowing of the polymer on exposure to light.
German Published Application DAS 1,201,544 describes flameproof molding compositions which have been rendered flameproof by the addition of 25-40% by weight of chlorinated hexamethylbenzenes, based on the polymer composition, in mixture with oxygen compounds or sulfur compounds of an element of Main Group V of the Periodic Table. However, additives of this order of magnitude have a detrimental effect on the mechanical properties of the basic polymers.
Materials which have been found most generally effective for styrenic polymers include polyhalogenated benzene, diphenyl and diphenyl ethers. These products, frequently in conjunction with metallic based co-catalysts, are generally functionally effective but are not completely satisfactory for various reasons. Some of the disadvantages of these materials include toxicity and environmental persistance. Thus, polychlorinated diphenyl and assorted brominated diphenyls build up in the liver and fatty tissue of animals with chronic physiological ill effects. Some of these materials have sufficient volatility that they tend to evaporate from the surface of molten plastic as it enters the mold. The vapor condenses on the surface of the mold and/or accumulates at the vents of the mold. This phenomenon is known as plate-out and manifests itself in one of two ways. If the vents become clogged, the part may actually burn as a consequence of compressing air and flammable vapors into an unvented mold. The high compression raises the temperature of the gases to the combustion point. The other and more common effect of plate-out is poor surface on molded objects since the plated out additive powder is pressed on to the surface of the part.
Frequent interruptions of the molding process required to physically wipe the molds free of plate-out.
Other deficiencies include incompatibility and volatility or decomposition temperatures which differ too much from the base polymer.
Another key deficiency of halogenated aromatic compounds, especially halogenated aromatic ethers, is the tendency to turn yellow when exposed to light. This is particularly bothersome in appearance parts where style is of great importance. A white or pastel colored appliance can turn yellow to brown after a short exposure to direct sunlight. Fluorescent lights, indirect lighting or incandescent lighting merely takes a little longer to discolor conventionally flame retarded styrenic plastics.
I have found that many of the deficiencies of currently available flame retardant systems may be alleviated by utilization of a combination of polyhalogenated diaryl carbonates and antimony oxide. The polyhalogenated diaryl carbonates were found to be useful fire retardants when used alone with certain high ignition temperature polymers (U.S. Pat. No. 3,382,207). However, these materials are ineffective when used alone with styrenic polymers. The explanation probably lies in the great difference in decomposition temperatures between the styrenic polymers and those of U.S. Pat. No. 3,382,207.
I have found that antimony oxide makes the difference and presume that its effect may be due to its ability to cause early (lower temperature) thermal cracking of the polyhalodiaryl carbonates.
Using this combination, I have found that volatilities are sufficiently low that plate-out is eliminated at styrenic polymer molding temperatures. The diaryl carbonates are biodegradable and do not persist as permanent residues. The appropriate flammability may be obtained. And finally, compositions containing my system yellow to a far less degree than the current standard of the industry.